This invention relates generally to a latch assembly which functions to latch a door to a door frame. More particularly, the present invention relates to latch assemblies which are employed in conjunction with exit bars to latch the exit door to the door frame.
Conventional latch technology to which the invention relates typically focuses on a door latch bolt which is projectable and retractable to provide the latching/unlatching function. The specific latch bolt operation and configuration is typically a compromise between durability, reliability and security considerations. For example, latch bolts which linearly slide and project into an associated strike of a door frame provide a high degree of latch security. However, this type of latch bolt is susceptible to wear due to the sliding interaction and consequently presents reliability concerns. In other applications, the latch bolt rotates or pivots into engagement with the corresponding door strike. While the pivoting motion of the latch bolt reduces the frictional wear characteristics normally associated with the sliding-type latch bolts, the tapered or contoured surface required for pivotal engagement provides a depth of engagement within the door strike which is somewhat easy to defeat and, consequently, does not provide an optimum level of security. Because of the intensive usage that exit bars and exit doors generally experience, especially in public facilities, exit bars ordinarily adopt the rotary-type latch bolt, compromising increased security for lessened wear.
While it is certainly possible to provide a door latch mechanism which has both rotary and slidable operative features, thereby optimizing both the wear and security characteristics, such a door latch mechanism requires a number of moving parts and can be relatively expensive to manufacture.
In addition, there are regulatory codes governing door opening specifications which must be met. These codes specify relatively small force requirements for door latching and unlatching. Typically the latch bolt requires an angled or beveled bolt geometry to retain reliable functionality given these mandated force requirements. The angular geometry chosen is a compromise however. As the angle increases, for example, the force required to retract the latch bolt decreases due to the increase in the resulting tangential force applied to the bolt. However, by incorporating a relatively large angle, when the door is heavily loaded, the strike and door tend to separate. This leads to the door opening under substantially large loads and resulting in an unsecured condition. At the other extreme, if there is no angle placed on the latch bolt, the door and strike are not prone to disengage, but the forces needed to retract the latch bolt are significant and may exceed those specified by code. Consequently, the purely linear retraction required for a latch bolt with no angle is ordinarily not desirable to meet the mandated door opening force requirements.
Briefly stated, the invention in a preferred form is a latch assembly for an exit door. A latch frame mounted to the secured side of a door includes a latch cover which defines a latch face opening. A latch bolt is mounted to the frame for pivotal movement between a position wherein a tip portion of the latch bolt projects from the opening, and a position wherein the tip portion is retracted. The tip portion includes a planar locking surface and an opposing curved or beveled engagement surface. The latch bolt is biased toward the extended position by a torsion spring. The latch bolt is secured in extended position by engagement with a blocking link mounted within the latch frame.
Preferably, the blocking link comprises a crank having first and second ends. The crank is pivotally mounted to the latch frame at a crank axis intermediate the crank ends. A force applied to the first crank end, such as by an exit bar, causes the crank to pivot out of engagement with the latch bolt, thereby allowing the latch bolt to rotate toward the retracted position.
A frame bolt is mounted to the door frame in opposing relationship with the latch bolt when the door is closed within the door frame. The frame bolt is biased away from the door frame to a projected position and displaceable towards the door frame. The frame bolt includes a planar locking surface and an opposing engagement surface which is preferably curved or tapered.
Preferably, the frame bolt is displaceably mounted in a plunger assembly, which is in turn mounted to the door frame. The plunger assembly may include a lock pin displaceably mounted adjacent the frame bolt locking surface for movement between a biased position projecting from the door frame and a position inward of said projected position toward the door frame. When the lock pin is in the projected position, the frame bolt is free to move toward the inward position. When the lock pin is displaced inwardly, the frame bolt is fixed in the projected position.
A jamb plate may be mounted to the door frame. The jamb plate spans the distance between the door frame and the latch cover when the door is closed. The jamb plate functions to cover the latch bolt when the door is in the closed position and as a guard when the door is in the open position.
When the door is in the closed position, the latch bolt is prevented from moving by the blocking link. The frame bolt may be prevented from moving by the lock pin in some embodiments. The latch bolt and frame bolt planar locking surfaces are in opposing surface-to-surface contact, thereby securing the door in the closed position. When the blocking link is moved, as by a force exerted on an exit bar operably connected to the blocking link, the latch bolt is freed for rotation to the retracted position. Continued exertion of force against the exit bar or door forces the latch bolt to move past the frame bolt and the door to be opened. Upon removal of the force exerted on the exit bar, the latch bolt and blocking link return to their biased positions so that the latch bolt is again fixed in the extended position. As the door returns to the closed position, the latch bolt displaces the frame bolt inwardly and moves past the frame bolt. When the latch bolt moves past the frame bolt, the frame bolt is biased into the projected position wherein each bolt locking surface is in opposing surface-to-surface contact, securing the door in the closed position.
An object of the invention is to provide a new and improved latch assembly especially adapted for incorporation into an exit bar.
Another object of the invention is to provide a new and improved latch assembly which is reliable under intensive usage and provides a high level of security.
A further object of the invention is to provide a new and improved latch assembly which incorporates the reliability features of a rotary latch and the high security features of a sliding latch bolt.
Other objects and advantages of the invention will become apparent from the detailed description and the drawings.